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Re: [LTO] Move ChangeLog.lto into the GCC sub directory

On Sat, 3 Jun 2006, Kenneth Zadeck wrote:

> > The next step will be to create the LTO "front end".  At first, it will
> > do nothing at all.  Then, it will start reading declarative information
> > out of object files.  Then, the ordinary compiler will be changed to
> > emit information for use at LTO.  Then, the LTO front end will start
> > reading the bodies of functions.  Finally, there will be oodles of work
> > to make it robust, support more programming languages, etc.  I'm not
> > sure there's anything that other people can usefully contribute here,
> > yet -- but there will be once we get the skeleton in place, which will
> > hopefully happen very soon.
> > 
> > I don't think think the IPA branch work is likely to conflict, at least
> > not any time soon.  In general, LTO will just be using the same
> > optimizers we already have; if those get smarter, LTO will get smarter
> > too.  The LTO branch will be primarily about reading/writing information
> >  and combining that information at link-time, not about improving the
> > underlying optimizations.
> > 
> The plan is to have the generation/insertion point be before the ipa
> passes.  This way we can take advantage of all of that work.  

Now my expectations for LTO were that we would have a (possibly reduced)
set of local optimization passes including an early inlining phase run
before the write-out for LTO;  IPA passes and "real" inlining at link
time followed by another set of local optimization passes.

This would make it possible to defer instrumentation for profile-based
optimizations to before the LTO write-out and have a clean re-start
for the profile-based transformations at the point of a profile directed
link-time only optimization phase.


Richard Guenther <>
Novell / SUSE Labs

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